The present invention relates to ionically conductive materials useful for electrochemical cell electrolytes or the like, and particularly to a new fused salt system which is molten and exhibits good conductivity at low temperatures.
Pentavalent metal fluorides are very low-melting, hygroscopic compounds which readily form eutectics with other fluoride salts. Many of these are liquid at or near room temperatures. See, for example, Ray W. Thoma, Advances in Molten Salt Chemistry, Vol 3, Chapter 6 (phase diagrams of binary and ternary fluoride systems). The melting temperatures of SbF.sub.5, TaF.sub.5, and NbF.sub.5 are all below 100.degree. C., with SbF.sub.5 exhibiting a boiling point below 150.degree. C.
Unfortunately, the reactivity of these fluoride salts with water makes them very difficult to work with. Accordingly, few practical applications involving all-fluoride systems have been pursued.
P.sub.2 O.sub.5 has recently been found to be useful in the formation of oxyfluoride glasses exhibiting very low softening temperatures. U.S. Pat. No. 4,314,031 discloses examples of such glasses, based on combinations of SnF.sub.2 and P.sub.2 O.sub.5, which exhibit glass transition temperatures below 100.degree. C.